1. Field of the Invention
This invention relates to apparatus for electronic signal processing and more particularly to improved analog-to-digital conversion by apparatus for providing wide-range current-to-frequency conversion in circuits with very-low input currents on the order of 1 .times. 10.sup.-14 A which flow in only one direction.
2. Description of the Prior Art
A common problem in constructing various electronic instruments is the quantization of a continuously-varying current or voltage so that subsequent digital data-processing systems can generate useful information based on such analog inputs. This analog-to-digital conversion becomes particularly difficult when the inputs vary over a wide dynamic range, such as 10.sup.7 :1, and when high accuracy must be maintained for very small currents, approaching 10.sup.-14 A. Unique techniques must be employed whenever circuit simplicity, low-power consumption and a low cost are also important.
Small input currents varying over a wide range are commonly produced by many devices such as, for example, ionization chambers used for radiation dosimetry or gas chromatography, photomultiplier tubes viewing light sources of widely-varying intensities, electrodes for collecting free ions or electrons in mass spectrometers or electrostatic analyzers, and large-value resistors for high-impedance voltage measurements.
The output signals from a quantizer handling such input currents generally enter digital data-processing circuits, which may in turn also control the quantizer. These circuits can perform such functions as determining and possibly displaying the instantaneous or average frequency which is proportional to the input current, the total number of pulses produced in a defined time interval which is proportional to the total charge (equal to the integral of the current) applied to the input during the interval, the time during which the frequency and thus the input current exceeds some fixed or adaptive threshold, or any other function commonly performed by digital or analog circuits.
A common application for this type of quantizer involves a small open ion chamber used in a radiation dosimeter which must be small, portable and capable of operating accurately for an extended period of time from batteries. Such an ion chamber will typically produce currents between 3 .times. 10.sup.-14 A and 1.8 .times. 10.sup.-7 A for the expected range of dose rates (1 mR/min to 6000 R/min) and total charges in a 1.2-s interval from 3.6 .times. 10.sup.-14 C to 2.4 .times. 10.sup.-8 C, corresponding to a total dose from 0.02 mR to 13 R. A simple, low-power converter with a minimum of controls and adjustments must quantize these currents for processing by subsequent circuits employing mostly CMOS digital integrated circuits.
U.S. Pat. No. 3,921,021, whose inventor is also one of the applicants herein, describes an improved wide-range current-to-frequency converter for use in circuits with input currents as low as 10.sup.-14 A. The system of circuitry operates as an analog-to-digital converter to produce an output frequency proportional to input current. The current-to-frequency converter enables digitization of an input signal by producing a train of discrete output pulses with a repetition frequency proportional to current applied to the input of the converter. The circuit utilizes a low-leakage charge-sensitive amplifier, a gated multivibrator, a charge pulser and a capacitive divider. The gated multivibrator under the control of the charge-sensitive amplifier at the input of the converter produces discrete pulses, which in turn cause the charge pulser to generate discrete units of charge, which are reduced in magnitude by the capacitive divider to become the charge-feedback pulses applied to the input of the charge-sensitive amplifier. This amplifier compares the feedback current consisting of repetitive charge-feedback pulses with the input current to the converter, and controls the gated multivibrator so that the pulse repetition rate varies in an appropriate manner to keep the feedback current equal to the instantaneous value of the input current, resulting in the repetition frequency of the gated-multivibrator pulses becoming a digital representation of the analog input current. This technique can provide a dynamic range of 10.sup.7 :1 and can handle input currents as small as 10.sup.-14 A directly without preceding electrometer amplifiers, whenever the input stage to the charge-sensitive amplifier contains a pair of MOSFETs. The circuitry also provides a mechanism for discharging the capacitive divider through a restoration diode at the converter input in a manner such that the circuit automatically establishes its own zero level.
The converter described in U.S. Pat. No. 3,921,021 has an embodiment which is intended for input currents flowing in one direction only. Such a unipolar converter is considerably simpler than the bipolar versions, and, because many of the devices generating signal currents are inherently unipolar, it is clearly advantageous to use the simpler unipolar converter with them. In this type of converter, however, because charges applied to the converter input in the opposite sense to that of the normal input current will not be nullified during the discharge period of the capacitive divider, the converter will accumulate an opposite-polarity offset corresponding effectively to a suppressed zero whenever such charges or currents are present without a significant normal input current.
For the sake of clarity in description only, consider the case where the normal input current flows from a signal device toward the converter, wherein this direction is defined as "positive." For this case the opposite-polarity offset arises because the capacitance at the converter input becomes charged negatively with respect to its proper reference voltage, and, until positive signal current flows for a sufficiently long time to recharge this capacitance back to the normal reference voltage, a unipolar converter will not produce output pulses indicative of the flow of signal current. Clearly the same principle applies to the opposite case, and this invention is not limited to such positive signal-current flow.
Such negative offsets can arise from several sources. For example, the offset voltage at the converter input could be positive with respect to nearby conductors, and current flow through leakage resistances to these conductors is in the negative sense. The input restoration diode provides one such leakage path, as may the gate-drain resistance of the input MOSFET. Alternatively, the relative values of the gate-source voltages of the two MOSFETs at the converter input may change slightly with time or temperature. If as a result the input voltage for current balance in the charge-sensitive amplifier rises, then effectively a negative charge has been added to the input capacitance, resulting in an undesirable negative offset. Finally negative charges can be introduced through the converter input itself by such nonstandard operations as connecting and diconnecting the signal-generating device or by resetting the converter in the presence of high noise levels, where the converter essentially sets its operating baseline at the peaks of noise-produced transients producing a negative offset.